The present invention relates to fabricating a photomask used in a photolithography process and, more specifically, to a method and apparatus for removing an edge bead from a photomask.
A photomask may be used to transfer a pattern onto a semiconductor wafer. The pattern which is to be transferred onto the wafer may be formed on a material, such as glass or quartz, which is substantially transparent. The material of the photomask may also include thin films of metal or other nontransparent material that prevent light from passing through selected portions of the photomask. In typical photomasks, the nontransparent material is opaque chrome.
Due to limitations imposed by the wavelength of light used to transfer the pattern, resolution degrades at the edges of the patterns of the photomask. Phase-shifting photomasks increase the resolution of patterns by creating phase-shifting regions in the transparent areas of a photomask. One type of phase-shifting mask is known as an “attenuated” or “half-tone” phase-shift mask. In this type of mask, the transparent regions transmit approximately 100% of the incident radiation, similar to the strong phase-shifting mask. However, the transparent areas are in-phase. The “phase-shifting” feature of an attenuated phase-shift mask is due to the fact that the opaque regions of the hard shifters are replaced with areas that are slightly transmissive, and act to shift light transmitted by 180°. Transmission of this incident radiation is typically in the 6–9% range for these masks.
Depending on the process used, an attenuating phase-shift mask may only require a single maskwriting step (similar to a binary mask). However, other processes may be used which require two or more maskwriting steps in order to create desired features, such as an opaque border region. At the present time, single-write processes, without the opaque border, have not been successful in photolithography steppers which use longer wavelengths for alignment or reticle identification, due to the material constraints of the pattern material.
Several methods of fabricating a photomask having an opaque layer on the border of the mask are known in the art. One process requires two mask-writing steps. First, a primary mask pattern is written and etched to the underlying substrate of the mask blank, the photoresist is stripped, and the mask blank is re-coated. Second, an opaque layer is written onto the border of the mask.
One drawback of the two mask-writing steps process is that the first chromium etch removes all exposed chrome. The subsequent lack of exposed chrome makes obtaining a good ground for the second e-beam write difficult without extensive write tool modification. Therefore, there is a need for a two mask-writing steps process that facilitates grounding the e-beam for the second mask-writing step.